Edward J. Kibblewhite

The Giant Magellan Telescope adaptive optics program

The Giant Magellan Telescope (GMT) adaptive optics (AO) system will be an integral part of the telescope, providing laser guidestar generation, wavefront sensing, and wavefront correction to every instrument currently planned on the 25.4 m diameter GMT. There will be three first generation AO observing modes: Natural Guidestar, Laser Tomography, and Ground Layer AO. All three will use a segmented adaptive secondary mirror to deliver a corrected beam directly to the instruments. The Natural Guidestar mode will provide extreme AO performance, with a total wavefront error less than 185 nm RMS using bright guidestars. The Laser Tomography mode uses 6 lasers and a single off-axis natural guidestar to deliver better than 290 nm RMS wavefront error at the science target, over 50% of the sky at the galactic pole. The Ground Layer mode uses 4 natural guidestars on the periphery of the science field to tomographically reconstruct and correct the ground layer AO turbulence, improving the image quality for wide-field instruments. A phasing system maintains the relative alignment of the primary and secondary segments using edge sensors and continuous feedback from an off-axis guidestar. We describe the AO system preliminary design, predicted performance, and the remaining technical challenges as we move towards the start of construction.

MCAO for Gemini South

The multi-conjugate adaptive optics (MCAO) system design for the Gemini-South 8-meter telescope will provide near-diffraction-limited, highly uniform atmospheric turbulence compensation at near-infrared wavelengths over a 2 arc minute diameter field-of-view. The design includes three deformable mirrors optically conjugate to ranges of 0, 4.5, and 9.0 kilometers with 349, 468, and 208 actuators, five 10-Watt-class sodium laser guide stars (LGSs) projected from a laser launch telescope located behind the Gemini secondary mirror, five Shack-Hartmann LGS wavefront sensors of order 16 by 16, and three tip/tilt natural guide star (NGS) wavefront sensors to measure tip/tilt and tilt anisoplanatism wavefront errors. The WFS sampling rate is 800 Hz. This paper provides a brief overview of sample science applications and performance estimates for the Gemini South MCAO system, together with a summary of the performance requirements and/or design status of the principal subsystems. These include the adaptive optics module (AOM), the laser system (LS), the beam transfer optics (BTO) and laser launch telescope (LLT), the real time control (RTC) system, and the aircraft safety system (SALSA).

A CLOSE COMPANION SEARCH AROUND L DWARFS USING APERTURE MASKING INTERFEROMETRY AND PALOMAR LASER GUIDE STAR ADAPTIVE OPTICS

We present a close companion search around 16 known early L dwarfs using aperture masking interferometry with Palomar laser guide star adaptive optics (LGS AO). The use of aperture masking allows the detection of close binaries, corresponding to projected physical separations of 0.6-10.0 AU for the targets of our survey. This survey achieved median contrast limits of ΔK ~ 2.3 for separations between 1.2λ/D-4λ/D and ΔK ~ 1.4 at 2/3λ/D. We present four candidate binaries detected with moderate-to-high confidence (90%-98%). Two have projected physical separations less than 1.5 AU. This may indicate that tight-separation binaries contribute more significantly to the binary fraction than currently assumed, consistent with spectroscopic and photometric overluminosity studies. Ten targets of this survey have previously been observed with the Hubble Space Telescope as part of companion searches. We use the increased resolution of aperture masking to search for close or dim companions that would be obscured by full aperture imaging, finding two candidate binaries. This survey is the first application of aperture masking with LGS AO at Palomar. Several new techniques for the analysis of aperture masking data in the low signal-to-noise regime are explored.

Sparse matrix wave-front estimators for adaptive-optics systems for large ground-based telescopes

A closed-loop adaptive-optics servo system is an iterative optical–digital processor that can be characterized by a first-order difference equation. This identification leads to a new class of wave-front estimators, some of which are extremely sparse and permit real-time subaperture intensity weighting.

Observation of optical pumping of mesospheric sodium.

We have observed a large variation with laser polarization in the amount of laser light resonantly backscattered from the Earths mesospheric sodium layer located at a 90-km altitude. This variation is evidence of optical pumping of mesospheric sodium atoms.

Design and field tests of an 8-W sum-frequency laser for adaptive optics

The design and characteristics of an 8 watt diode laser pumped version of the sum frequency laser designed for astronomy is described together with an outline of field test undertaken on the NOAO Vacuum Telescope Tower at Sac Peak. Optical pumping effects were shown to increase the return by a factor of 2.5 subarcsecond beacons were produces with a brightness equal to a 9.2 magnitude star with 1.2 watts incident at the sodium layer under sodium abundance column density of 5 X 109 atoms/cm2.

The Giant Magellan Telescope phasing system

The 25 m Giant Magellan Telescope consists of seven circular 8.4 m primary mirror segments with matching segmentation of the Gregorian secondary mirror. Achieving the diffraction limit in the adaptive optics observing modes will require equalizing the optical path between pairs of segments to a small fraction of the observing wavelength. This is complicated by the fact that primary mirror segments are separated by up to 40 cm, and composed of borosilicate glass. The phasing system therefore includes both edge sensors to sense high-frequency disturbances, and wavefront sensors to measure their long-term drift and sense atmosphere-induced segment piston errors. The major subsystems include a laser metrology system monitoring the primary mirror segments, capacitive edge sensors between secondary mirror segments, a phasing camera with a wide capture range, and an additional sensitive optical piston sensor incorporated into each AO instrument. We describe in this paper the overall phasing strategy, controls scheme, and the expected performance of the system with respect to the overall adaptive optics error budget. Further details may be found in specific papers on each of the subsystems.

Implementation of the Chicago sum frequency laser at Palomar laser guide star test bed

Work is underway at the University of Chicago and Caltech Optical Observatories to implement a sodium laser guide star adaptive optics system for the 200 inch Hale telescope at Palomar Observatory. The Chicago sum frequency laser (CSFL) consists of two pulsed, diode-pumped, mode-locked Nd:YAG lasers working at 1.064 micron and 1.32 micron wavelengths. Light from the two laser beams is mixed in a non-linear crystal to produce radiation centered at 589 nm with a spectral width of 1.0 GHz (FWHM) to match that of the Sodium-D2 line. Currently the 1.064 micron and 1.32 micron lasers produce 14 watts and 8 watts of TEM-00 power respectively. The laser runs at 500 Hz rep. rate with 10% duty cycle. This pulse format is similar to that of the MIT-Lincoln labs and allows range gating of unwanted Rayleigh scatter down an angle of 60 degrees to zenith angle. The laser system will be kept in the Coude lab and will be projected up to a laser launch telescope (LLT) bore-sited to the Hale telescope. The beam-transfer optics, which conveys the laser beam from the Coude lab to the LLT, consists of motorized mirrors that are controlled in real time using quad-cell positioning systems. This needs to be done to prevent laser beam wander due to deflections of the telescope while tracking. There is a central computer that monitors the laser beam propagation up to the LLT, the interlocks and safety system status, laser status and actively controls the motorized mirrors. We plan to install a wide-field visible camera (for high flying aircraft) and a narrow field of view (FoV) IR camera (for low-flying aircraft) as part of our aircraft avoidance system.

Reconstruction of discontinuous light-phase functions

The reconstruction of discontinuous light-phase functions is of major importance in adaptive optics. An efficient and simple algorithm that can reconstruct large arrays of phases from phase differences is presented. We prove that the algorithm yields a perfect result in the absence of noise, and we describe the function that it maximizes. We suggest a method that makes use of the reconstructed phase to measure the position of branch points. A simulation of the reconstruction of a 33x33 phase array is presented.

Facilitizing the Palomar AO laser guide star system

We describe the work that has gone into taking the sodium Laser Guide Star (LGS) program on the Palomar AO system from a successful experiment to a facility instrument. In particular, we describe the operation of the system, the BTO (beam transfer optics) system which controls the path of the laser in the dome, the aircraft safety systems and the optical systems which allow us to take advantage of the unique properties of the macro/micro pulse laser. In addition we present on sky performance results that demonstrate K-band Strehl ratios of up to 48%